In recent years, various image display devices such as a portable game machine, a mobile phone, and a personal digital assistant (PDA), which can be carried around and used by a user and include a display screen for presenting the user with various kinds of information, have appeared in the market (see, for example, Patent Document 1). Such a portable image display device executes various kinds of information processing according to the user's operation input and displays results thereof on the display screen. Patent Document 1: US 2007/0202956 A1